In recent years, semiconductor devices have been under high integration. When many highly-integrated semiconductor devices are arranged in a horizontal plane and are connected by wirings for final fabrication, there are problems due to the increase in wiring length, wiring resistance and wiring delay.
Under the circumstances, a three-dimensional integration technique for stacking semiconductor devices in three dimensions has been proposed. This three-dimensional integration technique uses a bonding system to bond two semiconductor wafers (hereinafter abbreviated as “wafers”) together. For example, the bonding system includes a surface modifying device (surface activating device) for modifying bonding surfaces of the wafers, a surface hydrophilizing device for hydrophilizing the surfaces of the wafers modified by the surface modifying device and a bonding device for bonding the wafers having the surfaces hydrophilized by the surface hydrophilizing device. In this bonding system, the surface modifying device modifies the surfaces of the wafers by plasma-processing, and the surface hydrophilizing device hydrophilizes the surfaces of the wafers by supplying pure water onto the surfaces. Thereafter, the two wafers are disposed in the bonding device so as to vertically face each other (hereinafter, the wafer existing at the upper side will be referred to as an “upper wafer” and the wafer existing at the lower side will be referred to as a “lower wafer”). The upper wafer is held by an upper chuck and the lower wafer is held by a lower chuck. These wafers are bonded to each other by a Van der Waals force and hydrogen bonding (an inter-molecular force).
The lower chuck has, e.g., a flat plate shape, and adsorptively holds the lower wafer on the entire upper surface thereof. However, there may be, e.g., a case where particles adhere to a rear surface of the lower wafer thus held, or a case where particles exist on a front surface of the lower chuck. As such, the front surface of the lower chuck becomes uneven (has a large flatness). In this case, such a flatness of the lower chuck is transferred to the lower wafer. This causes vertical distortion in a bonded overlapped wafer which is obtained by bonding the lower wafer and the upper wafer together.
In addition, if the front surface of the lower chuck is not flat, there may be a location where a distance between the upper surface and the lower wafer bonded together is short. In this location, when the upper wafer and the lower wafer make contact with each other, it is impossible to completely discharge air existing between the upper wafer and the lower wafer. This causes a void in the bonded overlapped wafer. Accordingly, there is room for improvement in the bonding process of the wafers.
As a result of the earnest research conducted by the present inventors, it was found that the existence of particles in an outer peripheral portion of a front surface of a second holding unit (used as the lower chuck) is a major culprit in generating a vertical distortion (a distortion exceeding a tolerance) in an overlapped substrate obtained by bonding a first substrate (or first wafer) and a second substrate (or second wafer). The present inventors found that the vertical distortion of the overlapped substrate is suppressed by making the contact area of the second holding unit with the outer peripheral portion of the second substrate small.